Lymph node blood vessels bring cells, oxygen, and micronutrients to the lymph node and are a critical component of a functioning immune system. During immune responses, the lymph node vasculature undergoes a rapid proliferative expansion followed by vascular maturation. While vascular expansion is characterized by unregulated endothelial cell proliferation, VCAM-1 expression, and HEV trafficking efficiency along with disrupted perivascular fibroblastic reticular cell organization, the subsequent maturation phase is characterized by reversal of these phenomena, thus promoting vascular quiescence and stabilization. Recent studies have begun to delineate the mechanisms involved in the induction of lymph node vascular growth during immune responses, but the mechanisms that regulate the subsequent period of vascular quiescence and stabilization and the functional importance of this process are not well understood. This application proposes to test the hypothesis that a dendritic cell subpopulation regulates vascular quiescence and stabilization in lymph nodes and that this vascular regulation is important for optimal immune responses. We will test the hypothesis via the following specific aims: 1) Delineate the mechanism by which vascular quiescence and stabilization promotes optimal immune responses. We will examine whether B cell survival factors are more limiting in lymph nodes. 2) Delineate the molecular mediators that regulate vascular quiescence and stabilization. We will use of mice that allow for conditional deletion of genes in CD11c+ cells. 3) Delineate the mechanisms by which FRC organization is regulated. We will use a combination of in-vitro and in-vivo techniques to examine the regulation of FRC phenotype and its relationship to FRC function. These studies will yield novel information on dendritic cell function, the regulation of vascular function, and the relationship between vascular regulation and immune function and may identify novel targets for autoimmune and lymphoproliferative diseases.